''Sanctions Shift Signals by Howe'' -- The Times. ''Howe Signals Policy Shift on Sanctions'' -- The Guardian. ''Sir Geoffrey Signals a Shift in Britain's South African Policy'' -- the European Wall Street Journal. The Daily Telegraph story said the British foreign secretary had ''signaled the government's more flexible line'' with the prime minister sitting quietly by his side.

Flexible? Impossible, you say -- not Margaret Thatcher in retreat, under fire. Not the ''Iron Lady'' who broke a bitter British coal strike and won the Falklands War. Yet that was the message in one day's headlines only two weeks after a flurry of Thatcher interviews had shown her to be moralistic, melodramatic, unyielding.

You have to wonder how the longest-serving, most experienced political leader in the Western world became caught up in this political and diplomatic donnybrook. All at once she seems at war with the Commonwealth, at odds with Buckingham Palace, at apparent cross-purposes with her foreign secretary, out of step with most of Britain's European partners and in the pocket, so to speak, of the commercial self-interests of a minority of her party.

The short explanation is that appearances outrun reality; the ''shift'' is illusory. Thatcher's isolation is more tactical than real. A careful reading of her various pronouncements in recent months leaves her more running room than you might suppose.

Semantics and sloppy shorthand have confused the issue. The catchword ''sanctions'' has been taken to mean everything from withdrawal of military attache's and embargoes on arms and computer sales to South Africa (already enacted with Thatcher's approval) to the European community's standby package of boycotts on coal, steel and gold coins and a clamp on new investment. None of this does Thatcher ''exclude.'' She is almost certainly prepared to accept all of it in due course under the euphemism of ''further measures.''

This popular catchword covers ''general, punitive sanctions'' that Thatcher has been shouting out against. These would be comprehensive, to include all of South Africa's significant trading partners and commercial connections, culminating conceivably in a naval blockade that would be impossible to arrange -- or apply.

But appearances matter. That is why Thatcher's performance of late may be more properly the concern of American policy makers assigned to worry about how the United Kingdom fits into established concepts of nuclear deterrents and dependable Atlantic partnership. Thatcher's policies fit the concepts nicely. Those of her principal opposition, the Labor Party do not.

So American strategists must have a care for the political fortunes of the Conservatives -- which were bleak enough before the uproar over South Africa. It understates things to say that the South African brouhaha has not helped. On the contrary, it has reinforced a developing perception that the Thatcher government may already be a burnt-out case, with general elections possible next spring and certain to be held no later than 1988.

This collapse is more astonishing when you consider the prime minister capped her initial election victory in 1979 with a landslide reelection in 1983 that gave her a mammoth parliamentary majority. Three years later, the Conservatives lag badly behind the aggregate opposition of Labor and the Alliance (a loose combination of the old Liberal Party and the new Social Democrats who broke away from Labor five years ago).

A full understanding of what has happened would require a deeper delving into the British social and political condition than is necessary for the point at hand: whatever the cause -- be it an excess of arrogance and insensitivity or simply tired Tory blood -- Thatcher seems to be squandering her most important political asset.

It is the exact opposite of the asset of her co-ideologist in Washington. Ronald Reagan rules with a laid-back charm that almost effortlessly invites trust and confidence. Thatcher rules best by the sense she conveys of discipline, command, control.

If that is what is slipping away, let it be noted that Thatcher's second best asset remains intact. She is blessed and buttressed by an opposition whose general disarray gives disharmony in the Democratic Party in the United States the look of solidarity. So Thatcher & Co. cannot be too readily written off.

But neither can her South African performance be disentangled from her domestic prospects. It is likely to weigh far more heavily on the internal affairs of Great Britain than on the internal affairs of South Africa.